1973 Z That Has Idle Issues

[kray-z-1]

It seems that my Z can not settle on a normal idle. Sometimes it backfires through the carbs - other times it afterfires at the exhaust. There doesn't seem to be any minor adjustments - it either one or the other. The PO had dual webers, I changed to SU's (3 screw). I have done the following:

1) installed 3 screw su's - using a rebuild kit from Ztherapy. I have taken them apart and reinstalled so many times...thinking about using wing nuts instead of bolts.

2) I have adjusted the valves - they are dead on - so don't think the problem is with the valves.

3) I installed an E80-12 dizzy and coil (removed ballast resistor). I have new plugs (NGK) and new wires.

If I get the idle to remain relatively stable at 800 rpm - taking it up to 3000 rpm seems smooth - but when trying to drive it - it bogs and backfires at the carb. Then the idle moves from 700 to 1300 rpm before settling on new idle rpm.

It just seems to have no power. When shifting, I get a fair amount of afterfire through the exhaust. When I adjust the idle adjust nut, its either too lean or too rich - can't seem to find a happy medium.

I have read su posts and elec dizzy posts until i'm nearly blind. I have noticed that the front damper seems to move faster than the rear damper (both up and down). Is this normal?

I have to be missing something - any ideas? (If I had hair....i'd be pulling it out!)

[kcoke]

Is the timing set accuretly.....If its back fireing its probably the timing. Does it have a timing advance.....Is it properly connected to the carb...?

[beandip]

I don't think it is ignition but could be . I am thinking carburetion. When you went through the SUs did you set or alter the float levels? If they are set to high you could get a rich mixture that you cant control with the mixture adjustment, under the carb. Try this , with the engine at idle and the air cleaner opened up exposing the intakes. Use your finger and lift a vacuum piston in the throat of one of the SUs and see how it effects the idle. If the engine continues to run as before then the opposite carb is too rich. Then do this with the second carb. If the engine dies when the vacuum piston is lifted , this means the other carb is set too lean. So to recap you lift the piston of one carb , this disables that carb, causing the engine to run on the second one . You should vary quickly tell if you have a vary rich setting. When I bought my Z the PO had one set lean, one rich, and my timing was set 10# retarded. By the way with the ZX ignition set the timing at 15*BDC if you haven't yet. I would like to hear what you find. Gary:rambo:

[kray-z-1]

I have to admit I am leaning toward timing. Since I have had the car, it has never had smooth power and has always back and or after fired.

I have checked the floats several times, as well as taken the su's apart and recleaned & checked everything. I have done the raising of the vacuum piston test - and have made the adjustments to where I thought it would be good. I balanced both front and back - yet during the drive I continue to get both backfire and afterfire - with no real power. It's like it has 2 personalities....one at idle in the driveway....and the other on the road.

This brings me to 2 questions:

1) How do you time the engine to get to 15 degrees when the pointer on the pulley doesn't go that high. I am assuming that the 15 degrees is without the vacuum advance connected. I am using the ol fashion non adjustable timing light. The dizzy seems to be situated correctly as far as rotor pointing to #1 cylinder.

2) Would any of the symptoms indicate timing to be too retarded....or too advanced?

I havent checked to see the timing mark is correct.....I wonder if the timing got jumped somewhere along the line - as it has never ran well since I got it.

The best I have been able to do is about 1000 rpm at idle - any rpm lower really causes dead spots in the acceleration ( usually causing backfire at the carb)

I really appreciate your input.....cause I have no clue. I remember buying a new 1972 z.....didn't have these problems on the original...

thanks again

[Walter Moore]

I don't know much about the 73, but the timing marks on my 71's front pulley are in 5 degree increments from 20BTDC to 20ATDC. (I hope.)

I can't get more than about 12BTDC because I have my distributor drive off by one tooth, but there are two marks past that.

(The 73, being more throttled by emissions regulations may be different.)

[JeffMopar]

Not sure if this will help, but when I originally picked up my Z it would fire out of the exhaust when accelerating and out the intake when decelerating. It actually lit the air cleaner element on fire before I fixed it! I used a Uni-Syn to synchronize the carbs and the problem disappeared. It was running almost entirely on the rear carb. Also check the oil level in the carbs. If the level is too high or low it will cause the vacuum pistons to not move equally.

Jeff

[beandip]

I have been mulling this around in my head most of the day. This is out of left field but check your PVC valve. Wash it out is some thinner and make sure it is not sticking open. If you have a big vacuum leak like this you wont be able to control the idle. In your first post you said that at 800 rpm to 3K it was smooth but when you drove, putting the engine under load, there was loss of power. This sounds more like timing , like being to retarded. Backing up here. When you first bought the Z , and before you started doing anything to it. How was it running? You added the ZX dist.? Is this when the problems started? You went over the carbs, is this when the problems began? I am just trying to find a correlation between what has been done and the problem at hand. Incidentally the markings on the timing indicator show up to 20 degrees BTDC , as was stated the markings are 5* apart. Now if all of these problems started when the new dist was installed then there may be a problem with the centrifugal advance . A broken spring or something like this. What weight oil is in the carb dampeners? Not knowing what all has been done since the problem started and or prior to it going on makes this difficult to figure out. I could be a combination of things. If the carbs are balanced and the mixture is set correctly and you assembled them correctly as you stated . Just another thought , connect the timing light and watch what the advance does from idle as you slowly raise the RPM. See if it is a smooth transition. Gary

[kray-z-1]

Gary

Ever since I got the z - there have been backfire & afterfire issues. I attributed it to the weber set up. I installed the su's - got the rebuild kit from ztherapy - checked the float level 3 different times - in fact went over the adjustment with Bruce from ztherapy the last time. After each time - the z ran virtually the same way as far as having problems getting the carbs to balance and not stall during driving. I then installed the ZX dizzy and coil this spring. Off the top of my head - i cant remember what the timing was set at - I'll check that tomorrow (Tuesday). I am using 20 wgt fork oil for the dampers (being from Milwaukee - using Harley fork oil).

When I'm checking the timing tomorrow - I'll watch the advance during the acceleration from idle.

I agree - difficult to determine once so many things have changed - especially when it never ran right since I got it. I'll also clean the plugs as they are now probably fouled from the various carb configerations. Hopefully Tuesday I'll have a better handle on the timing situation. It drives me crazy to know what the car is capable of....when tuned correctly.

Thanks for the help!

[kcoke]

Do you have the timing advance vacuum hose hooked to the left carb? Very important to have the vacuum line connected.

[kray-z-1]

I've adjusted the timing. It was at about 10 degrees - I changed that to 15 degrees. That seems to have eliminated both backfire at the carb and afterfire at the exhaust.

I set the fuel mixture to 2 1/2 on each su with the air mixture to 1 1/2 turns initially.

If I raise the rear piston - my understanding the car is now running on the front su - it wants to die no matter how much I richen the front carb. When I raise the front piston, idle is steady.

That would indicate to me - the front su isn't getting enough gas. Right?

I visually checked the nozzle when the fuel mixture was totally screwed in - the top of the nozzle is flush with the botton of the su body.

Can I assume the problem is only with the front su? Can I also assume the problem is with the float level?

I think we are getting closer to a solution - whatever that might be!

Thanks again for the help

[beandip]

Are you attempting to balance the carbs by just counting the revolutions of the adjustments? You need to balance the air mixture with a Uni-sun or similar tool that is measuring the air flow into the carbs. After the carbs are balanced air flow wise then start on the mixture adjustment . When you have this correct then go back and check the balanced again. Starting with the fuel mixture adjustment at 2 1/4 to 2 1/2 is a good starting place and the engine should run. and Idle . I start with the idle at about 700 rpm to 750. But trying to adjust the fuel mixture with the balance a skew is a waist of time. When you raise the front piston and the engine is running smoothly , this is telling you the REAR carb is rich. You disable the carb by lifting the piston. So the engine is running on the other carb. YES to answer your question you are getting closer. Have you done a compression check yet. Not to put a fly in the ointment. But a burnt valve can effect this stuff . but do balance the carbs properly and things will go smoothly for you. Good going sticking with this for as long as you have. It is encouraging knowing that you are not giving up. reread post #3 :classic:Gary

[a7dz]

I had the same problem as soon as I rebushed the butterfly shafts the idle smoothed out. It was leaking just enough through the shaft to effect the idle.